What do I do with the half I didn’t use?

Half an avocado, brown by morning. A block of cheese with a green bloom on the cut face. Half a can of tomato paste growing a skin for the fifth time this year. Seven pages in this library are about the half that gets thrown away, and an eighth is about the date on the packet that sends food to the bin. The seven agree on a small number of moves, and all eight carry more safety lines than you might expect from a question about leftovers, so those are kept whole here.

Keep the air off it and put acid on it. Press the film onto the guacamole is built from what many people shared about a cut avocado. Many people say it is air that browns the flesh, and many say citrus slows it, so everything else on that page is a way of getting between the flesh and the air: cling film pressed flat onto the surface so no air sits between film and food, the cut face laid on a plate, the two halves pushed back together, the lime first. The pit only shields what it touches, some people point out. Many people keep a cut half under water overnight and the page does not, because one person and the US Food and Drug Administration, in a 2022 statement reported by a US news outlet, say the water is where bacteria grow. The FDA was answering a fashion for keeping whole avocados under water for weeks: any Listeria or Salmonella on the skin can multiply in the water, and its research found Listeria can move into the flesh within fifteen days of refrigerated submersion. The FDA’s produce page also says to wash all produce under running water before cutting, even when you will not eat the skin, so the knife does not carry dirt from the skin into the flesh. One caution on that page keeps everyone honest: no method is perfect, air gets in, and the difference is between a thin brown skin and a brown half.

Cheese goes mouldy from your hands before it goes mouldy from the wrap. Keep your fingers off the cheese draws on what many people shared. Some people say touching the cheese leaves oils and bacteria on it that speed the spoilage, and one person says that matters more than which wrap you use; some people point out the shop wrapper left on inside a bag traps moisture and bacteria against the cheese, so throw it away and wrap the block fresh. Paper or plastic is the argument that page does not settle: many people say paper so the cheese can breathe, some answer that cheese counters heat-seal plastic and a bag lasts weeks, and what decides it, people say, is the cheese, hard or soft, how long it is being kept, and whether the plastic is sealed tight or left loosely rolled. Mould already on the block is a gap there, so the page carries the US Department of Agriculture’s guidance as widely reproduced today, because its own page could not be reached, and the words of the director of the University of Georgia’s Center for Food Safety, and the rule is worth reading whole. For a hard cheese, cut off at least an inch, 2.5 cm, around and below the mould spot, keep the knife out of the mould so it does not drag spores across the fresh cut, and rewrap in new paper or a new bag; throw the whole thing away if the mould covers the surface or the piece is too small to cut an inch below it. Soft cheeses, which that page lists as cottage, cream cheese, ricotta, brie and camembert, and any shredded, crumbled or sliced cheese with mould on it, are thrown away, not trimmed, because the moisture lets the mould grow deeper and spread its toxins further. The fridge line is the authorities’ too: at or below 40°F, 4°C, in the FDA’s words, and between 0 and 5°C in the UK government’s. The grater trick on that page is thirty minutes of chill, not a night in the freezer; some people point out frozen-solid cheese crumbles under the grater.

Freeze the rest in spoon-sized portions. Zest before you juice, then freeze it in cubes comes from many people with soft lemons, blackening herbs and half a can of paste, and for each of the three, some people freeze it in portions. Lemon juice into an ice-cube tray, then into a bag, because the page says juice left in an open tray takes freezer burn and off-flavours; some people point out zest first, because a squeezed lemon is too soft to zest, and one person says juicing first is the common mistake. Herbs chopped and topped with olive oil or melted butter in the same tray, one cube per pan. Tomato paste in tablespoon heaps on a sheet of baking or wax paper, frozen flat and broken into a bag; one correction on that page explains why the can was always too big, because paste is concentrated and a recipe wants a tablespoon or two. Many people cut fruit that is about to turn, spread it on a tray so the pieces freeze apart, and use it in smoothies in place of ice. One line stands above the method there, checked for the page: the page warns that garlic, and to a lesser degree herbs, sitting in oil at room temperature can grow the organism that causes botulism, and US food-safety guidance says homemade garlic-in-oil must be kept in the fridge and used within a few days, or frozen, never left out at room temperature. And one caution: do not store tomato paste or other acidic food against aluminium foil, because the acid eats through it and the food picks up the metal.

Stale is not the same as dry, so put the water back. Wet the bread before it goes back in is built from what many people shared. Some people revive hard bread with a spray, a mug of water standing beside it or a damp paper towel and a short burst of heat, because staling is the starch giving up its moisture; some say the oven does it better than the microwave, and that argument is carried both ways, with what decides it being what you are reviving and whether you want the snack this minute or the texture back properly. Microwaved bread has to be eaten at once; one person warns that it turns near-inedible once it cools again. Soft crisps need no water: tip them out onto a plate, twenty to thirty seconds, and let them cool before you judge them. Past reviving, many people dry it for crumbs and croutons and five soak it for French toast or bread pudding, because dry bread drinks the custard. The cautions on that page are about fire, and they belong here whole. Never microwave the crisp bag itself, because it can spark or shrink. Do not microwave heavily seasoned chips; one person says a cumin-dusted batch caught fire. Keep recycled paper towels out of the microwave, because they can carry tiny metal flecks that spark. Take the twist tie off the bread bag before anything goes near the microwave. And mouldy bread is a different thing from stale bread: the one person that addresses it says soft foods like bread with mould on them go in the bin whole, and the page carries no trimming rule for bread.

Cook the whole bag, then freeze what you cooked. Cook the whole bag, then freeze it draws on many people with a big cheap bag of potatoes in a warm kitchen. Some people say the way through it is to cook a large batch, mashed or baked are the two they name, and freeze it, so the week becomes reheating instead of peeling. The uncooked ones want somewhere cool, dark and dry, around ten degrees Celsius in one person’s detail, and away from bananas, onions, apples and avocados, which hurry them along. On the fridge, the page names both sides because the world moved: US food-safety guidance still says do not refrigerate raw potatoes, because the cold turns some of the starch to sugar and fried potatoes then form more acrylamide, and UK food-safety guidance changed in 2023 after its own research and now says the fridge is fine. Two safety lines sit above that page. Dry beans, which some people put with a potato to make it a meal, carry a natural toxin that boiling destroys; public food-safety guidance in the US and the UK names the kidney bean, says raw or undercooked ones cause severe vomiting and diarrhoea within hours, and says to soak them, throw the water away, and boil them hard, at least thirty minutes in the US guidance and at least ten in the UK’s, before they go into a slow cooker or a stew, because a slow cooker on its own may not get hot enough and undercooking can make the beans worse. And some people say a diet of only potatoes runs out of vitamin B12; the page is for stretching a bag, not for living on one food. Food-safety guidance adds that the green and the sprouts on a potato contain solanine, so cut them away and throw out any potato that is very green or tastes bitter.

Set honey has not gone off, and the jar goes in warm water, not boiling. Warm the jar, don’t boil it exists because one person says people throw away honey that has gone cloudy and hard, and some people put it plainly: pure honey crystallises, and one person adds that it is, if anything, a sign of the real thing. Many people stand the glass jar in warm water and wait; a correction, and an argument the people behind it settled, says boiling is counterproductive because it makes the honey set again faster. Two lines on that page are not preferences. Never heat honey in its plastic squeeze bottle, not in the microwave, not in hot water, not in a dishwasher with a heat cycle, because many people warn that the plastic can leach or melt; decant into glass first, and glass has its own rule, no sudden temperature change, or the jar cracks. And never give honey to a baby under one year old, including honey stirred into food, because honey can carry spores that cause infant botulism. Or, as some people say, do not bother melting it: set honey spreads, and some people prefer the texture.

The brine is an ingredient, once. Refill the pickle jar says the liquid left when the pickles run out is a finished, seasoned brine with at least one more job in it: washed, thin-sliced cucumber dropped in, the jar back in the fridge, and in two or three days there are new pickles, milder than the ones you bought. The envelope on that page is short and not optional. The refilled jar lives in the fridge and gets eaten within a week or two; the liquid gets reused once, not over and over, because every batch of vegetables waters it down and the acid and salt are what keep it safe; it never goes on a shelf and is never a canning liquid; and started clear but turned cloudy, fizzing, or smelling wrong means the bin, not a taste. The page also brines chicken in it, with one settled rule from those discussions that twenty-four hours is the ceiling.

Before you throw food out because of its date, check which kind of date it is: a best-before date is about quality and the food may still be safe after it, and a use-by date on chilled food is the one to take seriously. Check which date it is before you bin the food asks you to read its safety part first. Infant formula must never be used after its use-by date: it is the one food where the US government treats the date as a safety date. A baby, a pregnant person, an older adult and anyone with a weak immune system should go by the printed date on chilled foods and not trust smell. For these groups the US Food and Drug Administration and the US Department of Agriculture say to keep the fridge at 40°F (4°C) or colder, to eat chilled ready-to-eat food as soon as possible, and to avoid hot dogs and deli meats unless they are heated until steaming hot. For everyone, smell and taste can show that food has spoiled, but not whether it holds the bacteria that make people ill: the UK Food Standards Agency says food can look and smell fine after its use-by date and still cause food poisoning. Throw away a tin that is bulging or leaking, or that has a dented rim, without tasting it, and call 911 in the US or 999 in the UK if after tinned or home-canned food you get double vision, drooping eyelids, slurred speech, trouble swallowing or muscle weakness, and throw away bread and other soft food that has mould on it. If you get ill after eating: in the UK, call NHS 111 if you are worried about a baby under 12 months, have bloody diarrhoea, cannot keep fluids down, have had diarrhoea for more than 7 days or vomiting for more than 2 days, or if you are older or have a weak immune system and are ill with food poisoning; call 999 or go to A&E for vomit that is bloody or looks like coffee grounds, green vomit, a sudden severe headache or tummy ache, a stiff neck with pain in bright light, confusion or severe difficulty breathing. In the US, call your doctor if diarrhea lasts more than 3 days or has blood in it, you have a fever above 102°F, or you cannot keep liquids down; if you are pregnant and have a fever or flu-like symptoms, call at once. Call 911 for the signs in the 999 list above. Elsewhere, use your local emergency number.

The two governments use the same words differently. In the United States, the Department of Agriculture’s food safety service says that, apart from infant formula, dates are not an indicator of safety and federal law does not require them. In the United Kingdom, government guidance says use-by dates relate to safety and best-before dates to quality: you can eat food up to the end of the use-by date, but not after, unless it was cooked or frozen on or before it, and best-before dates sit on frozen, dried and tinned foods, and on cheese. Many people say makers set dates cautiously, so food is often good well past the printed date, and the US guidance adds that food that was mishandled, for example left out of the fridge for more than two hours, can grow bacteria before or after the date on the package. Many people say tinned food, dry grains, spices and hard cheese stay good for a long time past their dates when the packaging is intact; the packaging is the check, and a tin that is bulging, leaking or dented at the rim goes in the bin. For chilled food the date earns its place: the UK Food Standards Agency says the sniff test is not an appropriate way to check food with a use-by date, because the bacteria that cause food poisoning cannot be seen or smelled. UK government guidance says to keep chilled food at 5°C or below, and the US agencies say 40°F or below. If you cannot eat chilled food in time, freezing it on or before the use-by date stops it getting worse, and once defrosted it should be eaten within 24 hours; food cooked in time should be cooled, kept in the fridge and eaten within 48 hours. Cooked after the date, it is not made safe: the Food Standards Agency says not to eat, cook or freeze food after its use-by date. Soft foods with mould on them, bread included, go in the bin, because the mould’s roots go deeper than you can see; hard cheese is the exception, cut off at least an inch around and below the mould, keeping the knife out of the mould itself. That page is not permission to stretch a date for a baby, for anyone who is pregnant, an older adult or has a weak immune system, for anyone whose sense of smell or sight is not reliable, or when the fridge has been off or too warm, because a date assumes the food was kept cold. It covers the United States and the United Kingdom only.

Who this is not for, and what these pages leave open. The food pages cannot tell you how many days each method buys; “next day”, “a couple of days”, “weeks” and “months” are people’s own words, and the avocado, cheese, lemon and potato pages each name keeping time as a gap; the pickle page is the one that gives times, two or three days to new pickles and a week or two to eat them. If your cheese is brie, cottage or a bag of grated, the trimming line is not for you and the block goes. If you want lemon slices for a glass, the freezer is not your answer, because the flesh goes soft. If you are feeding a child under one, the honey page says the rule again on purpose. And if the half you cannot use is a symptom of not having enough for the whole, the second follow-up below is this library’s page for that.